Sources close to the bout confirmed the news with HeavyMMA.com on Wednesday afternoon. MMAWeekly.com issued the original report.
UFC 143 is headlined by an interim welterweight title fight between Nick Diaz and Carlos Condit. Werdum vs. Nelson is expected to occupy the co-main event slot for the card.
The newly-slim Nelson is coming off a win over Mirko Cro Cop at UFC 137, halting a two-fight losing streak to Junior dos Santos and Frank Mir.
Werdum left the UFC after a loss to dos Santos in 2008. He strung together three wins, including becoming the first man to legitimately defeat Fedor Emelianenko, after signing with Strikeforce. He lost to Alistair Overeem in a Strikeforce title fight in June.